Aligners are a series of tight-fitting custom-made retainers that slip over the teeth. Orthodontic aligners are typically used for patients who have mild or moderately crowded teeth, or have minor spacing issues. Patients who have severe crowding or spacing problems, a severe under-bite, a severe over-bite, or a cross-bite, may need more complex treatment. Once a dentist, orthodontist, or treatment professional decides how to correct the patient's bite, a plan is made for moving the teeth from the starting position to the final proposed position. A patient is fitted for several versions of aligners that make slight adjustments to move the teeth over the treatment time. Aligners are typically formed from a clear plastic or acrylic material to fit tightly over the teeth, but can be removed for eating, brushing, and flossing. Patients typically receive a new aligner every few weeks to continue moving the teeth into the desired position. Treatment time with an aligner is based on how much the teeth need to be moved or rotated. The more a bite is off or the more crooked the teeth, the longer the treatment. Treatment usually takes between ten and twenty-four months.
While aligners are gaining in popularity, they remain a relatively new technology in the field of orthodontia. As such, there is unfortunately a limited amount of research on orthodontic tooth movement using aligners. Most of the literature consists of case reports, editorials, blogs, or articles written by authors who may have a bias. There are very few evidence-based attempts to describe the type of tooth movement resulting from treatment with aligners. While there is no consensus in the field directed to an interpretation of the results of these studies, conventional thinking suggests that the movement is mostly uncontrolled tipping, with the center of rotation located between the center of resistance and the apex of the tooth.
FIGS. 1A and 1B illustrate a movement of a tooth in accordance with the conventional understanding of aligners in the field. As shown in FIGS. 1A and 1B, a tooth 2 is disposed in a gum tissue 4 at a first orientation. As a force is applied in the direction of Arrow A, tooth 2 rotates about a center of rotation 6 from the first orientation (FIG. 1A) to a second orientation (FIG. 1B). The rotation generally follows the path of Arrow B in FIG. 1B.
The orthodontic field is uncertain regarding what magnitude of sustained force is needed to move a typical tooth and what length of time the given force should be applied. Aligners are generally believed to move teeth 1 millimeter per month, without a clear understanding of whether this is optimal, efficient, or even correct across all patients. The industry standard is generally to apply a particular aligner to a set of teeth for two weeks. However, there is no evidence to suggest two weeks is the correct amount of time. Many practitioners believe the two week standard is heavily influenced by the material weaknesses of the first generation aligners, where the aligner would stretch and lose rigidity and force starting at about two weeks. However, more modern aligners do not suffer from similar losses of rigidity or inconsistent forces over time.
As shown in FIG. 2, the best results in tooth velocity or tracking are achieved when the periodontal pressure is in an optimal range 8. The optimal range 8 is known in the field and is about 1.5 N/cm2 to 2.6 N/cm2 of pressure on a given tooth. The optimal range 8 correlates to the amount of pressure needed to move a tooth while not harming the tooth, gums, or root in the process due to the applied pressure. If periodontal pressure is too low, the gum and root tissue will not react and the tooth will remain in place. If periodontal pressure is too high, root resorption may occur. Root resorption, is the breakdown or destruction and subsequent loss of the root structure of a tooth. This is caused by living body cells attacking part of the tooth. Severe root resorption is very difficult to treat and often requires the extraction of the tooth. Resorption may occur as a result of trauma, ectopic teeth erupting in the path of the root, chronic inflammation, excessive occlusal loading, or aggressive tumors, cysts, or other growths. However, the most common cause of root resorption in Western society is misapplied orthodontic forces.
Thus, ensuring aligners are within the optimal range 8 illustrated in FIG. 2 is extremely important, as catastrophic loss of a tooth is minimized, while tooth movement is maximized. As such, keeping aligners within the optimal range 8 provides major medical and economic advantages to the patient as well as the orthodontist or treatment professional. Unfortunately, there is a great deal of uncertainty in knowing the amount of force a given aligner is producing on a given tooth. Currently, there is no accurate way of measuring the force of an aligner on a tooth in vivo. Further, while the initial force is unknown, aligner forces may change over time in the oral environment due to deformation of the aligner or as a natural consequence of moving a set of teeth into a more desired position. Still further, the forces exerted on a tooth are complex and do not conform to a standard linear delivery. Thus, modeling or even approximating the force on a tooth from an aligner is extremely complex and there currently exists no accurate way to quantify the amount of force applied to teeth in aligners today.
One of the major drawbacks that may prolong the use of aligners in patients is non-compliance by the patient. In many cases, subjects who begin clear aligner treatment deviate from the programmed progression of aligners and require reevaluation, midcourse correction, and/or use of fixed appliances to achieve treatment goals. Not wearing an aligner as directed for twenty-two hours per day significantly slows the progression of treatment. Some treatment professionals believe that not wearing an aligner for one hour out of the twenty two hours as directed will require an additional twenty four hours of wear time to make up for the one missed hour. Other issues include miscalculation of treatment by the orthodontist or treatment professional, not selecting a good candidate for alignment treatment, and tooth movement stages not going as planned.
As of today, there exists no way to quantify the amount of force being applied to a tooth during aligner treatment. The orthodontist or treatment professional must rely on professional expertise, software, and visual inspection to predict and facilitate the treatment. A better understanding of the mechanics of tooth movement using aligners could lead to more appropriate selection of patients, better sequencing of tooth movement stages, and more efficient treatment. Thus, improved devices, systems, methods, and computer program products for quantifying the amount of force being applied to a tooth during orthodontic treatment of the tooth with an aligner are needed to provide a more efficient orthodontic experience for the patient as well as the orthodontist or treatment professional.